ORDERS 175 



or Ungulate, represented in the actual fauna by the Horses, 

 Rhinoceroses, Tapirs, Swine, and Ruminants. Almost as isolated 

 a> the Proboscidea anion,-; existing mammals are the few small 

 species constituting the family Hyracidce, and in their case palaeon- 

 tology affords no help at present, and therefore, pending further dis- 

 coveries, it has been thought advisable in most recent systems to 

 give them the honour of an order to themselves, under the name of 

 Hyracoidea. But the number of extinct forms already known allied 

 to the Ungulate, though not coming under the definition of either 

 of the two groups (Artiodactyla and Perissodactyla) under which all 

 existing species range themselves, is so great that either many new 

 orders must be made for their reception or the definition of the old 

 order Ungulate so far extended as to receive them all, in which 

 case both Proboscidea and Hyracoidea may be included within it. 

 Again, the Rodentia or gnawing animals — Rabbits, Rats, Squirrels, 

 Porcupines, Beavers, etc. — are, if Ave look only at the present state 

 of the class, most isolated. No one can doubt what is meant by a 

 Rodent animal, or have any difficulty about defining it clearly, at 

 least by its dental characters ; yet our definitions break down before 

 the extinct South American Typotkerium, half Rodent and half 

 Ungulate, which leads by an easy transition to the still more truly 

 Ungulate Toxodon, for the reception of which a distinct order 

 (Toxodontia) has been proposed. It has also been suggested that 

 the Rodents are connected by some of the extinct Tillodontia (or 

 Tseniodontia) with the Edentates. The Insectivora and the 

 Carnivora again are at present quite distinct orders, but they merge 

 into one another through fossil forms, and are especially connected 

 by the large group of primitive Carnivora, so abundantly repre- 

 sented in the Eocene deposits both of America and Europe, to which 

 Cope has given the name of Creodonta. The Carnivora also appear 

 to have been closely connected with the primitive Ungulates as repre- 

 sented by the extinct group called Condylarthra. In another 

 direction the step from the Insectivores to the Lemurs is not great, 

 and in past times the transition was probably complete. The Bats 

 or Chiroptera are allied to the Insectivora in all characters except the 

 extraordinary modification of their anterior extremities into wings; 

 but this, like the want of the hind limbs in the Cetacea and Sirenia, 

 makes such a clear distinction between them and all other mammals 

 that, in the absence of any knowledge of any completely inter- 

 mediate or transitional forms, they can be perfectly separated, and 

 constitute as well-defined an order as any in the class. We have, 

 however, an inkling of the mode in Avhich the Insectivora were 

 modified into Chiroptera shoAvn us by the so-called Flying Lemur 

 (Goleopithecus). Finally, Ave have the important and Avell-character- 

 ised group called Primates, including all the Monkeys and Man ; 

 and the question is not yet solved as to hoAv and through what 



